1. Field of the Invention
The present invention relates to a magnetic recording medium, a method of manufacturing the same, and a magnetic recording/reproducing apparatus using the magnetic recording medium.
Priority is claimed on Japanese Patent Application No. 2008-209907, filed Aug. 18, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, the application range of magnetic recording apparatuses, such as magnetic disk apparatuses, flexible disk apparatuses, and magnetic tape apparatuses, has increased remarkably, and their importance thereof has increased. Therefore, a technique has been developed for significantly improving the recording density of magnetic recording media used for these apparatuses. In particular, the development of an MR (magneto resistive) head and a PRML (partial response maximum likelihood) technique has accelerated the improvement in the areal recording density of magnetic recording media. In recent years, with the development of GMR (giant magneto resistive) heads and TuMR (tunneling magneto resistive) heads, the areal recording density of the magnetic recording media has increased significantly.
As such, there is a demand for a further increase in the recording density of magnetic recording media. In order to meet the demand, it is necessary to improve the coercivity, the signal-to-noise ratio (S/N ratio), and the resolution of a magnetic recording layer. In a longitudinal magnetic recording type that has generally been used, with the increase in linear recording density, recording magnetic domains adjacent to a magnetization transition region mutually weaken their magnetizations, which is called self-demagnetization. In order to prevent the self-demagnetization, it is necessary to reduce the thickness of the magnetic recording layer to increase shape magnetic anisotropy.
When the thickness of the magnetic recording layer is reduced, the strength of the energy barrier for maintaining the magnetic domain is substantially equal to that of thermal energy, and the phenomenon in which the amount of recorded magnetization is reduced due to temperature variation (thermal fluctuation phenomenon) is not negligible. The thermal fluctuation phenomenon or the self-demagnetization determines the limit of the linear recording density.
In recent years, a technique has been examined for solving the problem of the reduction in thermomagnetic and improving the linear recording density of the longitudinal magnetic recording type. For example, a medium having an AFC (anti-ferromagnetic coupling) structure has been proposed in order to maintain the magnetization state of the magnetic domain at a high level.
As a technique for further improving areal recording density, a perpendicular magnetic recording technique has attracted attention. In the longitudinal magnetic recording type according to the related art, a medium is magnetized in the in-plane direction. However, in the perpendicular magnetic recording type, a medium is magnetized in a substantially perpendicular direction to the surface of the medium. In this way, in the perpendicular magnetic recording type, it is possible to avoid the self-demagnetization that prevents an increase in linear recording density in the longitudinal magnetic recording type. Therefore, it is considered that the perpendicular magnetic recording type is suitable for high-density recording. In addition, since the perpendicular magnetic recording type can maintain the thickness of the magnetic layer to be constant, it is possible to relatively reduce the effect of thermomagnetic relaxation caused in longitudinal magnetic recording.
In general, a perpendicular magnetic recording medium is obtained by sequentially forming an orientation control layer, a magnetic recording layer, and a protective layer on a non-magnetic substrate. In many cases, a lubrication layer is formed on the surface of the protective layer. In addition, in many cases, a soft magnetic underlayer, which is an underlayer, is provided below the orientation control layer. The orientation control layer generally includes a seed layer and an intermediate layer formed on the substrate in this order. The intermediate layer is formed in order to further improve the characteristics of the magnetic recording layer. In addition, the seed layer functions to align the crystal particles of the intermediate layer and the magnetic recording layer, and control the shape of a magnetic crystal.
The crystal structure of the magnetic recording layer is important to manufacture a perpendicular magnetic recording medium with good characteristics. That is, in the perpendicular magnetic recording medium, generally, the crystal structure of the magnetic recording layer is a hexagonal close-packed (hcp) structure. It is important that a (002) crystal plane is parallel to the surface of the substrate, that is, a crystal c-axis ([002] axis) is aligned substantially perpendicular to the surface of the substrate with the least possible disorder. However, the perpendicular magnetic recording medium has an advantage in that it can use a relatively thick magnetic recording layer, but it has disadvantages in that the total thickness of the layers of the medium is likely to be larger than in the longitudinal magnetic recording medium and the crystal structure is likely to be disordered during a process of forming thin films of a medium.
In order to align the crystal particles of the magnetic recording layer with the least possible disorder, the intermediate layer of the perpendicular magnetic recording medium has been made of Ru having the same hexagonal close-packed (hcp) structure as the magnetic recording layer. Since the crystal of the magnetic recording layer is epitaxially grown on the (002) crystal plane of Ru, a magnetic recording medium with good crystal orientation is obtained (for example, see Patent Document 1).
The seed layer disposed below the intermediate layer needs to have characteristics that improve the crystal orientation of the intermediate layer. Therefore, in the related art, the seed layer having a face-centered cubic (fcc) structure has been used (for example, see Patent Document 2). In this case, the (002) crystal plane having the hexagonal close-packed (hcp) structure is preferentially oriented on a (111) crystal plane having the face-centered cubic (fcc) structure. Therefore, it is possible to reduce the total thickness for obtaining the same orientation, as compared to when Ru is directly formed on the underlayer.
In Patent Document 3, a first intermediate layer (seed layer) formed immediately on the underlayer is made of Ti having the hexagonal close-packed (hcp) structure, a second intermediate layer is made of Cu-5at % Ti having the fcc structure, and a third intermediate layer is made of Ru.
However, it is necessary to increase the thickness of the seed layer having the hexagonal close-packed (hcp) structure to a certain value (10 nm or more), in order to improve the crystal orientation. However, when the seed layer is made of a non-magnetic material and the thickness of the seed layer is increased, the distance between the magnetic recording layer and the soft magnetic underlayer is increased. As a result, during recording, the magnetic field applied from the head is weakened, and the writing performance is reduced.
In contrast, an element having the face-centered cubic (fcc) structure shows a certain degree of crystal orientation with a thickness of about 5 nm. When the crystal orientation is improved, the crystal particle diameter thereof is also increased. Since one crystal of the intermediate layer and one crystal of the magnetic recording layer are grown on one crystal of the seed layer, an increase in the crystal particle diameter of the seed layer means an increase in the crystal particle diameter of the magnetic recording layer. In order to further improve areal recording density, it is crucial to improve the crystal c-axis orientation of the magnetic recording layer, make the particle diameters uniform, and reduce the size of the particles. When the seed layer having the face-centered cubic (fcc) structure is used, it is difficult to improve the crystal orientation and obtain fine particles with a uniform diameter.
As described above, the seed layer made of a crystal material having the face-centered cubic (fcc) structure or the hexagonal close-packed (hcp) structure is insufficient to obtain a perpendicular magnetic recording medium having good read/write characteristics. Therefore, a perpendicular magnetic recording medium is required which is capable of maintaining the writing performance during recording by reducing the thickness between the underlayer and the magnetic recording layer, improving the crystal orientation of the magnetic recording layer, obtaining fine crystal particles with a uniform diameter, and being manufactured by a simple process.
Patent Document 4 discloses a structure in which an orientation control film is made of a material having a C11b structure and the orientation control film is made of a material including at least one or two or more of Al, Ag, Au, Cu, Ge, Hf, Ni, Si, Ti, Zn, and Zr.
[Patent Document 1] JP-A-2001-6158
[Patent Document 2] JP-A-2002-109720
[Patent Document 3] JP-A-2005-190517
[Patent Document 4] JP-A-2004-46990
The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a magnetic recording medium which is capable of improving the perpendicular orientation of a perpendicular magnetic recording layer while maintaining the writing performance during recording and obtaining both an improvement in perpendicular orientation and fine magnetic crystal particles with a uniform diameter, and which enables information to be recorded or reproduced at high density, a method of manufacturing the same, and a magnetic recording/reproducing apparatus.